1. Technical Field
The invention relates to a phototransistor and a method of operation of such a phototransistor.
2. Description of the Related Art
Phototransistors are useful for detecting optical information in general and low light levels in particular.
A bipolar phototransistor is a classic example of a phototransistor that amplifies the photocurrent generated by incident light. The bipolar transistor has good output characteristics since the output is a high impedance current source which is easy to interface. Unfortunately it is difficult to make bipolar transistors with good performance in amorphous silicon or polysilicon which are suitable for so-called large area technologies, such as those used in imagers.
Thin film phototransistors do exist and a conventional thin film phototransistor is illustrated in FIG. 1. The structure is formed on a substrate 2, typically glass or plastic. A gate 4 is formed over the substrate, and the whole surface covered in turn with a gate insulator 6 and a layer of semiconductor 8. The semiconductor 8 acts as the photosensitive layer, and may typically be doped amorphous silicon (a-Si:H). A source contact 10 formed from a highly doped layer 12 and a metallic contact 14 is formed at one end, and a similar drain contact 16 is formed at the other end of the device also having a highly doped layer 12 and metallic contact 14. A protective layer 18, for example of nitride, protects the structure.
The resistance of the a-Si:H layer 8 is high in the absence of light and this high resistance reduces to a very low level the current passing through the transistor formed by source 10, gate 4 and drain 16.
When the device is illuminated, electron-hole pairs are created in the a-Si:H layer 8 and this very significantly reduces the resistance of the transistor thereby increasing the current when the transistor is switched on by applying a suitable voltage to the gate 4.
However, such prior art phototransistors are not very sensitive since no more than one electron and one hole is created per photon. This means that high light levels are required.
Thus, there is a need for an improved phototransistor capable of being integrated into large-area and/or thin-film circuits.